Electronic dryer control

ABSTRACT

A dryer control including sensing means to contact items being dried to establish a current path therethrough having a resistance which is a function of the moisture content of the items bridging the sensing means; switch means to terminate operation of the dryer; and circuit means, including a capacitor, interconnecting the sensing means and the switch means. The circuit means include an insulated gate field-effect transistor. The transistor operates the switch means to terminate operation of the dryer when the charge on the capacitor reaches a predetermined level.

I United States Patent [1113,621,293

[72] inventor Donald S. l-leidtmann [56] References Cited Louisville, 9-UNITED STATES PATENTS [21] P 3,331,139 7/1967 Finnegan et al. 34/48 x[221 3,436,838 4/1969 l-lelfrich 317/1485 B x [451 Paemed 1971 3 47193:; 10/1969 Elders 34/45 Assignee General p y Primary ExaminerDonald D.Forrer Assistant Examiner-L. N. Anagnos Attorneys-James E. Espe and F.H. Boos,.lr., Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman[54] ELECTRONIC DRYER CONTROL ABSTRACT: A dryer control includingsensing means to con- 12 Claims 4 Drawing Figs tact items being dried toestablish a current path therethrough [52] US. Cl. 307/252 N, having aresistance which is a function of the moisture content 34/44, 34/45,34/48, 307/252 W, 317/ 148.5 E, of the items bridging the sensing means;switch means to ter- 307/304 minate operation of the dryer; and circuitmeans, including a [51] l t.(l .H03kl7/64, capacitor. interconnectingthe sensing means and the switch F26b 13/ 10 means. The circuit meansinclude an insulated gate field-efi'ect [50] Field of Search 34/43, 44,transistor. The transistor operates the switch means to ter- 45, 48,134; 307/252 R, 252 A, 252 H, 252 N, 252 .l, 252 W, 279, 304, 284;317/l48.5 B

minate operation of the dryer when the charge on the capacitor reaches apredetermined level.

as HF 6'5 I 82' 93 t. 73 7| un- "55 SS 75 76 4,2 I 5% i fi- 7 97 78 77 LL/J e4 8o 4 63 3e 67 PATENTEnuuv 16 197i SHEET 1 UF 3 mvsmon. DONALD s.HEIDTMANN PIC-ll H is ATToRwEY BACKGROUND OF THE INVENTION Over the pastseveral years, clothes-dryer controls which use a measurement of theresistance of fabrics being dried as the control basis have beendeveloped. In the more popular approach to this type of control, sensorsin the dryer drum are positioned so as to be directly contacted by thefabrics being dried. In this manner, the fabrics become a part of thecontrol circuit and, in effect, serve as a variable resistance. Theresistance of the fabrics increases as the moisture content of thefabrics decreases. The sensors are connected across the capacitor andcontrol the voltage to which the capacitor charges. A neon lamp, orsimilar device, is associated with the capacitor such that when thecharge reaches the breakdown voltage of the lamp, means are initiated toterminate dryer operation. In such a control circuit, the neon lamp isthe least stable and least reliable component and it becomes quiteexpensive to obtain a neon lamp having the required close tolerances.

Moreover, other approaches to resistance-measuring controls haveincluded a relatively expensive timer mechanism. In these arrangements,the timer is energized when the fabrics reach some predeterminedmoisture content and continues to operate for several minutes to bringthe moisture content down to the desired level. In arrangementsheretofore proposed, it was necessary to utilize the timer in order topro vide a sufficient time delay, between charging of the capacitor anddeenergization of the dryer, to bring the moisture content of thefabrics down to the desired level.

Accordingly, it would be advantageous to provide a resistance-sensingdryer control which obviates the neon lamp but retains the desirableaspects of former control. It would also be advantageous to provide adryer control of the type referred to which obviates the relativelyexpensive timer mechanism but still provides the necessary delay betweenthe time the fabrics reach the predetermined moisture content and thedryer is shut off.

An object of this invention is to provide an improved dryer control ofthe resistance-sensing type.

Another object of this invention is to provide such an improved controlwhich obviates the heretofore utilized neon lamp.

Another object of this invention is to provide such an improved controlwhich obviates the timer mechanism.

SUMMARY OF THE INVENTION Briefly stated, in accordance with one aspectof this invention, there is provided a control for a dryer whichincludes switch means effective to terminate operation of the dryer inresponse to an electrical signal. Sensing means are positioned tocontact items being dried to establish a current path therethroughhaving a resistance which is a function of the moisture content of theitems bridging the sensing means. Insulated gate field-efiect transistormeans are provided having a source connection, a drain connection and agate connection. Circuit means are provided to establish a voltage atthe source connection. Other circuit means, including the sensing means,provides a voltage at the gate connection which varies with theresistance of the item bridging the sensing means. Circuit meansinterconnect the transistor means and the switch means. With thisarrangement, the transistor means will provide an electrical signal tosaid switch means when the sensing means provides a resistance whichcauses a voltage at the gate connection approaching the-value of thevoltage at ,the source connection.

BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes withclaims particularly pointing out and distinctly claiming the subjectmatter which is regarded as the invention, it is believed the inventionwill be better understood from the following description of thepreferred embodiments taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a side elevational view of a clothes dryer with which myimproved dryer control may be used, the view being partly broken awayand partly in section to illustrate details;

FIG. 2 is a schematic electric circuit diagram of one embodiment of myinvention;

FIG. 3 is a schematic electric circuit diagram of another embodiment ofmy invention; and,

FIG. 4 is a schematic electric circuit diagram of another embodiment ofmy invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis illustrated a domestic clothes dryer 10 which is provided in theusual way with a cabinet 11 having a front door 12 to provide access tothe interior of the cabinet for loading or unloading fabrics. Providedon the top wall 13 of cabinet 11 isa control panel 14 which may includea suitable control knob 15. By manual manipulation of control knob 15,the machine may be caused to start, and automatically proceed through acycle of operation. X

Within cabinet 11, there is provided a fabric-tumbling container, ordrum 16, mounted for rotation about a substantially horizontal axis.Drum 17 is substantially cylindrical in shape, having'a first outercylindrical wall portion 18; second and third, outer, cylindrical wallportions 19 and 20,. located respectively adjacent the front and back ofthe drum; a front wall 2I and a backwall 22. Outer wall portions 18, 19and 20 are imperforate over their entire length so that the outer-shellof the basket is imperforate. A plurality of clothes-tumbling ribs 23are provided ontheinterior of wall portion 18 so that fabrics arelifted'up by the-ribs when the drum rotates, and then tumbled back downto the bottom of the drum.

The front drum 1? may be rotatably supported within cabinet 11 bysuitable idler wheels 24,"one of which is shown in FIG. 1. These wheelsare rotatably secured to the top of a member 25 which extends up frombase 26' of the machine. The wheels 24 are disposed beneath the drum incontact with portion 19 thereof so as to support portion 19 on each sideto provide a stable support. The rear end of drum [7 receives support bymeans of a stub shaft 27 extending from the center of wall 22. Shaft 27is secured within a bearing 28 supported by a baffle 29 which, in turn,is rigidly secured to the backwall 30 of cabinet 11 by any suitablemeans such as welding at a number of points 31. Withlthe arrangementshown, the drum may rotate on a horizontal axis, with rollers 24providing the front support and stub shaft 27 within bearing 28providing the rear support.

In order to provide for the flow of a stream through the drum, ture 32inits front wall 21 and-with an opening in the form of a plurality ofperforations 33 in its rear wall 22, the perforations in the presentcase being fonned to extend around the rear wall in an annulus.

Bafile member29 also serves -to support heating means 34 which, in themachine illustrated, includes two electrical-resistance heating elements34a' and 34b, appropriately insulated from the baffle member. "Elements34a and 345 may be annular in shape so as to be generally coextensivewith perforations 33 in wall 22. A second bafi'le member 35 is rigidlysecured to the backwall 22 of the drum outside the ring of perforations33 and within the stationary baflle 29, so that an anof drying air nularair inlet 36 is, in effect, formed by baffles 29 and 35. In

this manner a passageis formed for air to enter annular inlet opening 36between the battles, pass over the heating-means 34, then pass throughopenings 37, formed in baffle'35, and then through the perforations 33to the interior of drum 17.

The front opening 32 of the drum is substantially closed by means of astationary bulkhead 38. Bulkhead 38 is made up of a number of adjacentmembers including the innerwall 39 of access door 12; a stationary frame40 for the door, fbrmed as a flange of the front wall .41 of thecabinet; the inner surface member-42 of an exhaust duct which isformedby cooperation of member 42 with the front wall 41 of the cabinet;and an annular flange 43 mounted on frame 40 and on the duct wall. It

the drum is provided with a central aper-.

will be noted that a suitable clearance is provided between the inneredge of drum opening 32 and the edge of bulkhead 38 so that there is norubbing between the drum and bulkhead during rotation of the drum. Inorder to prevent any substantial air leakage through opening 32 betweenthe interior and exterior of the drum, a suitable ring seal 44,preferably formed of feltlike material, is secured to flange 43 insealing relationship with the exterior surface of drum wall 21.

Front opening 32 also serves as a means whereby clothes may be loadedinto and unloaded from the drum. Door 12, whose inner wall forms part ofthe bulkhead closing the opening, is mounted on cabinet 11 so that, whenthe door is opened, clothes may be inserted into or removed from thedrum through the doorframe 40. It will be noted that the door includesan outer, flat, imperforate section 45 and an inwardly extending hollowsection 46 mounted on the outer section. Hollow section 46 extends intothe doorframe 40 when the door is closed, and the inner wall 39 whichcomprises part of the combination bulkhead 38 is actually the inner wallof the hollow section.

The air outlet from the drum is provided by a perforated opening 47formed in the inner wall 39 of the hollow door section 46. The bottomwall section of the door 12 and the adjacent wall of door frame 40 areprovided with aligned openings 48 and 49. Opening 49 provides theentrance to a duct 50 formed by the cooperation of member 42 with frontwall 41. As shown, a lint trap 51 is positioned in the exhaust duct 50at opening 49, the trap being supported by the doorframe 40.

Duct 50 leads downwardly to an opening 52 formed in the member 25 whichsupports the wheels 24. Opening 52 constitutes the inlet to a blowermember 53 contained within a housing 54 and directly driven by anelectric motor 55. An inlet, such as the opening 61, is provided so thatthe blower means draws ambient air in through opening 61, over theheating means 34, then through the basket, then through the door 12 andduct 50, and then into the blower. From the blower the air passesthrough an appropriate duct (not shown) out of cabinet 11 so as to beexhausted from the machine.

In addition to driving blower 53, motor 55 constitutes the means foreffecting rotation of drum 17. In order to effect this rotation, motor55 is provided with a shaft 56 having a small pulley 57 formed at theend thereof. A belt 58 extends around pullet 57 and also entirely aroundthe cylindrical wall section 18 of drum 17. The relative circumferencesof pulley 57 and wall section 18 cause the drum to be driven by themotor at a speed suitable to effect tumbling of the clothes therein. inorder to effect proper tensioning of belt 58, there may be provided asuitable idler assembly 59 secured on the same support 60 which supportsone end of the motor. Thus, air is pulled through the drum and at thesame time the fabrics in the drum are tumbled. When the air is heated byheating elements 340 and 34b, the heated air passing through the drumcauses vaporization of moisture from the clothes and the vapor iscarried off with the air as it passes out of the machine.

In order to sense the amount of moisture in the fabrics being dried inthe drum 17, and thus obtain a suitable signal for controlling theoperation of the dryer 10, sensing means are provided to contact and bebridged by the items being dried. In the machine of FIG. 1 the sensingmeans is in the form of a pair of spaced electrodes 62 and 63 which aremounted on an inwardly extending portion 64 of member 42. The extension64 is formed so that electrodes 62 and 63 are exposed to the interior ofthe drum and positioned to be readily contacted and bridged by thefabrics as they tumble within the drum 17. A temperature-sensitiveswitch or thermostat 65 is positioned in duct 50 just below opening 49to sense the temperature of the air leaving drum 17 and passing throughopenings 48 and 49.

The operation of dryer is controlled by the new and improved controlarrangement schematically illustrated in the circuit diagram of FIG. 2.Terminals 66, 67 and 68 are connected to a source of electrical power,specifically, 230-volt AC power. Terminal 67 is connected to the neutralconductor of the three wire supply so that a ll5-volt AC supply isprovided across terminals 66 and 67 while a 230-volt AC power supply isprovided across terminals 66 and 68. Terminal 66 is directly connectedto a conductor 69 while terminal 67 is directly connected to a conductor70. Motor 55 is connected between conductors 69 and 70 while heatingmeans 34 is connected between conductor 69 and terminal 68. A switch 71is connected in series with motor 55 and switch 72 is connected inseries with heating means 34 such that when either of these switches isopen, the component connected in series therewith is deenergized.Switches 71 and 72 form a part of a relay further including coil 73.Switches 71 and 72 are normally open and will remain open until coil 73is energized to move the switches 71 and 72 to their closed position.

Coil 73 is connected between conductors 69 and 70 in series with aresistor 74 and a controlled rectifier 75. Controlled rectifier 75 maybe a conventional silicon controlled rectifier having an anode 76,cathode 77 and gate 78. A capacitor 79 shunts coil 73 so that whencontrolled rectifier 75 is conducting, in response to a suitable signalapplied to gate 78, coil 73 will maintain switches 71 and 72 closeddespite the fact that rectifier 75 is providing only half-wave power tocoil 73. Thus, whenever controlled rectifier 75 is conducting, coil 73will maintain switches 71 and 72 in their closed positions. A capacitor80 may be connected across the anode to cathode terminals of rectifier75 to suppress any transient signals received from the power source.

Thus, the portion of the control circuit thus far described comprises aswitch means effective to tenninate the operation of the dryer inresponse to an electrical signal applied to gate 78 of controlledrectifier 75. This switch means includes controlled rectifier 75, coil73, and switches 71 and 72.

Also connected between conductors 69 and 70 is a resistor 81 and a Zenerdiode 82 in electrical series relationship with each other. Connectedbetween resistor 81 and Zener diode 82 is a conductor 83. Resistor 81reduces the voltage applied to conductor 83 and diode 82, while diode 82limits the voltage to which conductor 83 may rise due to the breakdowncharacteristics of Zener diode 82 which are well known in the art. Thus,half-wave rectified voltage is applied to conductor 83, due to therectifying characteristics of diode 82 when conductor 70 is positivewith respect to conductor 69, with the peak voltage of conductor 83,during its positive half cycle, being limited by the breakdowncharacteristics of diode 82.

A transistor 84 is provided and is of the insulated gate fieldeffecttype sometimesreferred to as a metal-oxide-semicom ductor transistor.Transistor 84 has a gate 85, source 86 and drain 87. The insulated gatefield-effect transistor is a voltageoperated device requiring little orno gate current. Moreover, the input resistance is exceptionally highbecause the gate behaves as a capacitor with very low leakage. In thecontrol circuit of FIG. 2, transistor 84 is a P-channel-type whereinwhen the gate voltage is more negative than the source voltage, thetransistor conducts from source to drain. As the gate voltage approachesthe source voltage, the transistor discontinues to conduct from sourceto drain. in most insulated gate field-effect transistors presentlycommercially available, the transistor shuts off or discontinuesconduction from source to drain when the gate voltage approaches within2 to 5 volts of the source voltage. Thus, in a control circuit of FIG.2, whenever the voltage of gate 85 is more than 5 volts more negativethan the voltage of source 86, transistor 84 will conduct from source 86to drain 87.

The voltage of source 86 is provided by a center tap variable resistor88 connected between conductors 83 and 70 such that the center tap 89provides a voltage-divider effect for source 86.

The voltage at gate 85 is established through a network including avariable resistor 90 which, in conjunction with a resistor 91, serves asa voltage divider to establish a voltage across the series-connectedresistor 92, diode 93, and capacitor 94. The sensing means, comprisingelectrodes 62 and 63, is

connected in parallel with capacitor 94 along with resistors 95 and 96.Drain 87 is connected to conductor 70 through a re sistor 97 and acapacitor 98 and to gate 78 of controlled rectifier 75. With thisarrangement, whenever transistor 84 is conducting from source 86 todrain 87, a signal is applied to gate 78 to render rectifier 75conductive so that coil 73 is energized and switches 71 and 72 remainclosed.

The function of the control circuit is, as mentioned above, to terminateoperation of the dryer in response to the moisture content-of the itemsbeing dried. When the items reach the desired moisture content, motor 55and heating means 34 are tobe deenergized, although these two componentsmay not become deenergized at the same instant. Thermostat 65, as shownin FIG. 2, shunts switch 71 so that motor 55 will be energized as longas either of thermostat 65 or switch 71 is closed. As mentioned above,thermostat 65 opens and closes in response to the temperature of the airleaving drum 17. in the embodiment of FIG. 2, thermostat 65 is anormally open thermostat and will open whenever the temperature it issensing drops below some predetermined value. Thus, when thepredetermined moisture content of the items being dried reaches thatlevel wherein coil 73 is deenergized and switches 71 and 72 are opened,motor 55 will continue to move air through drum 17 and continue totumble the items in drum 17 until the temperature of the air passingthermostat 65 drops below some predetermined value. Heating means 34will, however, be deenergized at the time that coil 73 is deenergized.

As the circuit in FIG. 2 is initially energized, the half-wave rectifiedpower provided by diode 82 establishes a source voltage on source86 oftransistor 84 by virtue of resistor. 88. At the time, a varying gatevoltage is applied to the gate 85 of transistor 84 through thevoltage-divider network comprising resistors 90, 91, 95 and 96 and theresistance of the items bridging electrodes 62 and 63. The gate voltageof gate 85 is variable in that the resistance of the items bridgingelectrodes 62 and 63 varies with the moisture content of those items. Itis the relationship of the gate voltage, applied to gate 85, to thesource voltage applied to source 86 which determines whether transistor84 will conduct to provide a suitable signal to gate 78 of rectifier 75.

The source voltage at source 86' is established by diode 82' andresistor 88. The gate voltage at gate 85 remains more negative than thesource voltage because the resistance of the items bridging electrodes62 and 63 will be relatively low as compared to the resistance ofresistor 92. Capacitor 94 will charge to a voltage representative of thevoltage across the series-connected resistors 95 and 96 and the itemsbridging electrodes 62 and 63. As the dryer continues to operate, andthe items become drier, the resistance of the items increases and thevoltage on gate 85 becomes less negative with respect to source 86. Thisprocess continues until such time as the voltage on gate 85 approachesto within 2 to 5 volts of the voltage at source 86 whereupon thetransistor means 84 "shuts ofi" or discontinues to conduct.

As mentioned above, while transistor 84 is conducting, a signalisapplied to gate 78 of. controlled rectifier 75 causing rectifier 75 toconduct during those half cycles when the polarity of the signal appliedto terminals 66 and 67 is proper. During the time that the gate 78 isreceiving the electrical signal and rectifier 75 is conducting, coil 73remains energized and switches 71 and 72 remain closed. When the voltageon gate 85 of transistor 84 approaches to within 2 to 5 volts of thevoltage on source 86, and transistor 84 discontinues conduction fromsource 86 to drain 87, the signal to gate 78 of rectifier 75discontinues and coil 73 is deenergized. Upon deenergization of coil 73,switches 71 and 72 open. The opening of switch 72'immediatelydeenergizes heating means 34 while the opening of switch 71 makes thecontinued energization of motor 55 dependent upon the condition ofthermostat 65.

l have found that a control as illustrated in FIG. 2 for present-dayfabric dryers may be made with the following components:

Resistor 74 600 ohms Controlled Rectifier 75 CIO6B Capacitor 79 20microfarads Capacitor 80 0.05v microfarad Resistor 81 27 K ohms ZenerDiode 82 IN968B Transistor 84 MEMSI lC Resistor 88 50 K ohms Resistor 9050 K ohms Resistor 9l 6!! K ohms Resistor 92 I00 megohms Diode 93 GP2-3l 2 Capacitor 94 3 microfurnds Resistor 95 v 470 K ohms Resistor 96 470K ohms Resistor 97 I0 K ohms Capacitor 98 0.! microtarad It can be seenfrom the=values listed above for resistor 92 and capacitor 94 that thecharging time constant is approximately 300 seconds, this representingthe product of the resistance of resistor 92, expressed in megohms, andthe capacitance of capacitor 94, expressed in microfarads. As long asitems of relatively high moisture content, and concurrently relativelylow resistance, bridge electrodes'62 and 63, capacitor 94 will dischargethrough the circuit including resistors 95 and 96 and the electrodes 62and 63 to prevent a full charge from-being established on capacitor 94untilthe fabrics or items-being dried reach the proper resistance.Capacitor 94 provides a timelag between the creation of high resistanceacross electrodes 62 and 63 and the deenergization of rectifier 75. Withsuch timelag, rectifier 75 will continue to conduct even though itemsmomentarilyfail to contact electrodes 62 and 63. if, as the fabricstumble, there are momentary lapses of contact with electrodes 62 and 63,the charging time of capacitor 94, resulting from the large timeconstant of the charging circuit, will provide a period during which theitems may reestablish contact before rectifier 70 is deenergized.

Moreover, because the time constant is in the magnitudeof severalminutes, the charging time of capacitor 94 also provides a period tobring the moisture content of the items being dried down to asubstantially lower level than the level at which their resistanceallows capacitor 94 to begin charging. In a normal resistance-measuringdryer control, the resistance of fabrics being dried reaches a magnitudeof 5 to l0 megohms when the moisture content-is approximately l5percent. It is difficult to measure a change in resistance when theitems have a moisture content less than 15 percent. However, themoisture content of 15 percent may not be as low as the desired moisturecontent and, for this reason, in systems heretofore proposed, a timingdevice was incorporated in the control circuit which wouldbe energizedat approximately l5 percent moisture content and then time out anadditional drying period to bring the moisture content of the clothesdown to the desired level. The relatively high time constant of thecharging circuit ofcapacitor 94,provides this additional drying periodwithout requiring the relatively expensive timer mechanism.

Resistor 90 is actual practice comprises a manually adjustablepotentiometer-which has its control knob accessible to the operator ofthe machine so that the operator, by adjusting resistor 90, can adjustthe ultimate dryness of the items being I dried by varying the chargingrate of capacitor 94 since capacitor 94 charges through resistor 90.Varying the re sistance of resistor will not vary the time constantestablished by resistor 92 and capacitor 94 but it will vary the voltageto which capacitor 94 charges and, since resistor 90 is positionedbetween capacitor 94 and the path through which the voltage at source 86is established, it does vary the voltage to which capacitor 94 will becharged relative to source 86. Accordingly, when the resistance ofresistor 90 is lowered, capacitor 94 will charge to aneffective voltagerelative to source-86 in a shorter time. Center justable so that afactory adjustment can be made to calibrate the voltage established atsource 86.

In many applications of an automatic clothes dryer, it is desired tohave a cool-down period following the removal of tap resistor 88-is madeadmoisture from the clothes to the desired level so that the clotheswill be cool when removed from the dryer. Experience has also shown thatwith permanent press items, it is especially desirable to have thedrying period followed by a cool-down period during which the clothesare tumbled without the application of heat. In order to achieve this inthe circuit of FIG. 2, thermostat 65 shunts switch 71 so that motor 55will continue to tumble the clothes within drum 17 after heating means34 has been deenergized. This tumbling action will continue until thetemperature of the air passing from drum 17 into duct 50 drops to somepredetermined temperature. When this occurs, thermostat 65 opens andmotor 55 becomes deenergized.

Referring now to FIG. 3, another embodiment of the present invention, isshown. Those components which are substantially identical to thecomponents of the embodiment illustrated in FIG. 2 have been givenidentical reference numerals. The operation of the switching means ofthe embodiment of FIG. 3 is the same as the operation of the switchingmeans of the embodiment of FIG. 2. That is, as long as controlledrectifier 75 is conducting, coil 73 is energized and switches 71 and 72are maintained in their closed positions to energize heating means 34and motor 55. Similarly, thermostat 65 shunts switch 71. It will benoted that gate 78 of controlled rectifier 75 is connected to thecollector connection 99 of a transistor 100. Transistor 100 also has anemitter 101 and a base 102. Collector 99 of transistor 100 is connectedto conductor 83 through a resistor 103.

An insulated gate field-effect transistor 104 has a source 105, a drain106 and a gate 107. Source 105 is connected to a voltage-divider networkcomprising resistors 108 and 109, with resistor 109 being a variableresistor to provide calibration of the voltage applied to source 105.Drain 106 is connected to conductor 70 through a resistor 110 and tobase 102 of transistor 100. Gate 107 is connected into a voltage-dividernetwork comprising resistors 111, 112, 113 and 1 14 as well as the itemsbridging electrodes 62 and 63. A capacitor 115 interconnects gate 107with conductor 83 and shunts the leg of the voltage divider comprisingresistors 113 and 114 and the items bridging electrodes 62 and 63.

The operation of the circuit of FIG. 3 is somewhat similar to thatdescribed above in connection with FIG. 2 in that as the resistance ofthe items bridging electrodes 62 and 63 increases, capacitor 115 isallowed to charge to a higher level to establish some predeterminedvoltage at gate 107 of transistor 104 to afi'ect the source to draincurrent flow through transistor 104. The change in this current flow inturn affects the signal applied to gate 78 of controlled rectifier 75.However, whereas in the circuit of FIG. 2 the transistor 84 conductedcurrent from source to drain until such time as the items becamerelatively dry, in FIG. 3 transistor 104 does not conduct until suchtime as the items become relatively dry.

Specifically, the operation of the circuit of FIG. 3 is as follows. Asthe circuit is initially energized, and items of relatively lowresistance are bridging electrodes 62 and 63, capacitor 115 is preventedfrom reaching a substantial charge. As a result, the voltage at source105 is such that the voltage at gate 107 is not sufficiently negativewith respect to the source voltage for transistor 104 to conduct fromsource 105 to drain 106. During this condition, transistor 100 remainsnonconductive from collector 99 to emitter 101. Accordingly, a signal isapplied to gate 73 through resistor 103 to maintain coil 73 energized.As the items bridging electrodes 62 and 63 become more resistive, andthe charge on capacitor 115 increases, the voltage at gate 107 oftransistor 104 becomes more negative with respect to the voltage atsource 105. Depending upon the calibration of variable resistor 109, atsome predetermined moisture content of the items bridging electrodes 62and 63, transistor 104 conducts from source 105 to drain 106 applying asignal to base 102 of transistor 100. As this signal is applied to base102, transistor 100 conducts from collector 99 to emitter 101 shuntinggate 78 and, in effect, removing the signal from gate 78 so thatcontrolled rectifier 75 no longer conducts. As controlled rectifier 75discontinues conducting,

coil 73 is deenergized and the eventuaj'sirutdbwn of the dryer takesplace as it does in the circuit of FIG. 2.

Examples of appropriate components for the circuit of FIG. 3 are setforth below:

Controlled Rectifier 75 C I068 Capacitor 0.05 microfamd Resistor 81 22 Kohms Zencr Diode 82 IN96HB Diode 93 GP2-3 l 2 Transistor 100 2N5l72Resistor 103 33 K ohms Transistor 104 TIXS67 Resistor 108 3300 ohmsResistor 109 20 K ohms Resistor H0 47 K ohms Resistor I11 20 megohmsResistor 112 $0 megohms Resistor 3 I00 K ohms Resistor 114 100 K ohmsCapacitor 115 5 microfarads Diode I16 IN5059 It should be noted that inthe embodiment of FIG. 3 a diode 116 is shown shunting coil 73 toprevent chatter of the relay during the half cycle of the applied powerwhen the polarity of that power is such that controlled rectifier 75 isnot conducting. Diode 116 serves the same purpose as capacitor 79 in thecircuit of FIG. 2.

In FIG. 4 there is illustrated a circuit diagram of another embodimentof the present invention similar to the e'inbodiment of FIG. 3. Similarcomponents have been given identical reference numerals. It will benoted that transistor 100 has been omitted and the output of transistor104 is directly applied to the gate of controlled rectifier 75.Moreover, controlled rectifier 75 is in series circuit with the heater117 of a thennally operated switch 118. Switch 118 includes switchelements 119 and 120, which are normally closed but which will openunder the influence of heat generated by heater 117. Thus, switchelements 119 and 120 will remain closed, energizing motor 55 and heatingmeans 34 until such time as controlled rectifier 75 conducts to energizeheater 117.

The operation of the circuit in FIG. 4 is similar to that of FIG. 3 inthat transistor 104 does not conduct until such time as the resistanceof the items bridging electrodes 62 and 63 reaches some predeterminedvalue corresponding to a degree of dryness of the items. When transistor104 conducts, a signal is applied to gate 78 of rectifier 75 causingrectifier 75 to conduct. Conduction of rectifier 75 causes heater 117 toopen switch elements 119 and 120. When switch element 120 opens, heatingmeans 34 is deenergized. When switch element 119 is opened, energizationof motor 55 becomes dependent upon the condition of thermostat 65.

Examples for appropriate components for the circuit of FIG. 4 are setforth below:

Controlled Rectifier 79 CIO6B Capacitor 80 0.05 microfarads Resistor 8133 K ohms Zener Diode 82 IN968B Transistor I04 TIXS67 Resistor 108 3300ohms Resistor I09 20 K ohms Resistor l0 K ohms Resistor 111 I0 mcgohmsResistor I12 20 megohms Resistor 113 100 K ohms Resistor I14 I00 K ohmsCapacitor 5 microfarads It should now be apparent that each of theembodiments of FIGS. 2, 3 and 4 provide a resistance-sensing clothesdryer control which retains many of the advantages of the systemsproposed heretofore, e.g., continued drying of the fabrics beyond thepoint where the moisture content triggers the control circuit to bringthe fabrics down to the desired level of dryness, a cool down feature toallow the drum to continue to tumble with cool air passing therethrough,and the feature wherein if the circuit is triggered by momentary absenceof fabrics bridging the electrodes, subsequent bridging of theelectrodes by moist fabrics will reset the circuit so that the circuitis not irreversibly locked in to termination of the drying cycle by themomentary absence of fabrics bridging the electrodes. Moreover, thesecircuits obviate the relatively expensive timer mechanism and alsoobviate the troublesome neon lamp common in systems heretofore. Anotheradvantage of the circuits of the present invention is that by employingan insulated gate field-effect transistor, very high resistances can beemployed in the charging circuit of the capacitor shunting theelectrodes to provide the very long time constant in the chargingcircuit of the capacitor. This is-achieved because the insulated gatefield-effect transistor is a voltage operated device requiring virtuallyno current into the gate.

As will be evident from the foregoing description, certain aspects ofthe invention are not limited to the particular details of constructionof the examples illustrated, and it is contemplated that othermodifications, applications or variations will occur to those skilled inthe art. it is therefore intended by the appended claims to cover suchmodifications, applications and variations as do not depart from thetrue spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is: I

1. A control for a dryer comprising:

switch means effective to terminate operation of the dryer upondeenergization of said switch means;

sensing means positioned to contact items being dried to establish acurrent path therethrough having a resistance which is a function of themoisture content of the items bridging said sensing means;

insulated gate field-effect transistor means having a source connection,drain connection and gate connection; circuit means establishing avoltage at said source connection; circuit means including said sensingmeans to provide a voltage at said gate connection which varies with theresistance of the items bridging said sensing means; and,

circuit means interconnecting said transistor means and said switchmeans;

whereby said transistormeans will deenergize said switch means when saidsensing means provides a resistance which causes a voltage at said gateconnection which approaches the value of the voltage at said sourceconnection.

2. The invention of claim 1 wherein said switch means comprises acontrolled rectifier in electrical series circuit with a switchmperatingdevice whereby said switch-operating device is energized in response toenergization of said controlled rectifier.

3. The invention of claim 2 wherein said switch-operating device is thecoil of a relay.

4. The invention of claim 2 wherein said switch-operating device is anelectric heater and the switch operated thereby is a thermallyresponsive switch.

5. A control for a dryer comprising:

shutoff means effective to terminate operation of the dryer upondeenergization of said shutoff means;

sensing means positioned to contact items being dried to establish acurrent path therethrough having a resistance which is a function of themoisture content of the items bridging said sensing means;

insulated gate field-effect transistor means having a source connection,drain connection and gate connection; circuit means establishing avoltage at said source connection;

circuit means including said sensing means, a capacitor and through saidresistance means and is discharged through said sensing means as long asitems of sufficiently low resistance bridge said sensing means;

the capacitance of said capacitor and the resistance of said resistancemeans having values to provide an R-C time constant of at least 1minute; and,

circuit means interconnecting said transistor means and said wherebysaid transistor means will deenergize said shutoff means to terminateoperation of the dryer when said capacitor is charged to a voltage whichapproaches the voltage at said source connection.

6. The invention of claim 5 wherein said shutoff means comprises acontrolled rectifier in electrical series circuit with aswitch-operating device whereby said switch-operating device isenergized in response to energization of said controlled rectifier.

7. The invention of claim 6 wherein said switch-operating device is thecoil of a relay.

8. The invention of claim 6 wherein said switch-operating device is anelectric heater and the switch operated thereby is a thermallyresponsive switch.

9. A control for a dryer comprising:

switch means effective to terminate operation of the dryer in responseto an electrical signal;

sensing means positioned to contact items being dried to establish acurrent path therethrough having a resistance which is a function of themoisture content of the items bridging said sensing means;

a capacitor;

resistance means connected in series circuit with said capacitor wherebysaid capacitor is charged through said resistance means;

said capacitor being connected in parallel circuit with said sensingmeans whereby the voltage to which said capacitor is charged dependsupon the moisture content of items bridging said sensing means;

said resistance means and said capacitor having resistance andcapacitance values, respectively, to provide an RC time constant of atleast 1 minute; and,

circuit means interconnecting said capacitor and said switch means toprovide an electrical signal to said switch means in response to thecharge on said capacitor reaching a predetennined level corresponding toa predetermined moisture content of items being dried;

said circuit means including an insulated gate field-effect transistorhaving its gate-drain path connected in parallel with said capacitor;

whereby said transistor will provide an electrical signal to said switchmeans when the charge on said capacitor reaches said predeterminedlevel.

10. The invention of claim 9 wherein said switch means comprises acontrolled rectifier in electrical series circuit with aswitch-operating device whereby said switch-operating device isenergized in response to energization of said controlled rectifier.

11. The invention of claim 10 wherein said switch-operating device isthe coil of a relay.

12. The invention of claim 10 wherein said switch-operating device is anelectric heater and the switch operated thereby is a thermallyresponsive switch.

* s a t-

1. A control for a dryer comprising: switch means effective to terminateoperation of the dryer upon deenergization of said switch means; sensingmeans positioned to contact items being dried to establish a currentpath therethrough having a resistance which is a function of themoisture content of the items bridging said sensing means; insulatedgate field-effect transistor means having a source connection, drainconnection and gate connection; circuit means establishing a voltage atsaid source connection; circuit means including said sensing means toprovide a voltage at said gate connection which varies with theresistance of the items bridging said sensing means; and, circuit meansinterconnecting said transistor means and said switch means; wherebysaid transistor means will deenergize said switch means when saidsensing means provides a resistance which causes a voltage at said gateconnection which approaches the value of the voltage at said sourceconnection.
 2. The invention of claim 1 wherein said switch meanscomprises a controlled rectifier in electrical series circuit with aswitch-operating device whereby said switch-operating device isenergized in response to energization of said controlled rectifier. 3.The invention of claim 2 wherein said switch-operating device is thecoil of a relay.
 4. The invention of claim 2 wherein saidswitch-operating device is an electric heater and the switch operatedthereby is a thermally responsive switch.
 5. A control for a dryercomprising: shutoff means effective to terminate operation of the dryerupon deenergization of said shutoff means; sensing means positioned tocontact items being dried to establish a current path therethroughhaving a resistance which is a function of the moisture content of theitems bridging said sensing means; insulated gate field-effecttransistor means having a source connection, drain connection and gateconnection; circuit means establishing a voltage at said sourceconnection; circuit means including said sensing means, a capacitor anda resistance means to provide a voltage at said gate connection whichvaries with the resistance of the items bridging said sensing means;said sensing means, said capacitor and said resistance means beinginterconnected so that said capacitor is charged through said resistancemeans and is discharged through said sensing means as long as items ofsufficiently low resistance bridge said sensing means; the capacitanceof said capacitor and the resistance of said resistance means havingvalues to provide an R-C time constant of at least 1 minute; and,circuit means interconnecting said transistor means and said shutoffmeans; whereby said transistor means will deenergize said shutoff meansto terminate operation of the dryer when said capacitor is charged to avoltage which approaches the voltage at said source connection.
 6. Theinvention of claim 5 wherein said shutoff means comprises a controlledrectifier in electrical series circuit with a switch-operating devicewhereby said switch-operating device is energized in response toenergization of said controlled rectifier.
 7. The invention of claim 6wherein said switch-operating device is the coil of a relay.
 8. Theinvention of claim 6 wherein said switch-operating device is an electricheater and the switch operated thereby is a thermally responsive switch.9. A control for a dryer comprising: switch means effective to terminateoperation of the dryer in response to an electrical signal; sensingmeans positioned to contact items being dried to establish a currentpath therethrough having A resistance which is a function of themoisture content of the items bridging said sensing means; a capacitor;resistance means connected in series circuit with said capacitor wherebysaid capacitor is charged through said resistance means; said capacitorbeing connected in parallel circuit with said sensing means whereby thevoltage to which said capacitor is charged depends upon the moisturecontent of items bridging said sensing means; said resistance means andsaid capacitor having resistance and capacitance values, respectively,to provide an R-C time constant of at least 1 minute; and, circuit meansinterconnecting said capacitor and said switch means to provide anelectrical signal to said switch means in response to the charge on saidcapacitor reaching a predetermined level corresponding to apredetermined moisture content of items being dried; said circuit meansincluding an insulated gate field-effect transistor having itsgate-drain path connected in parallel with said capacitor; whereby saidtransistor will provide an electrical signal to said switch means whenthe charge on said capacitor reaches said predetermined level.
 10. Theinvention of claim 9 wherein said switch means comprises a controlledrectifier in electrical series circuit with a switch-operating devicewhereby said switch-operating device is energized in response toenergization of said controlled rectifier.
 11. The invention of claim 10wherein said switch-operating device is the coil of a relay.
 12. Theinvention of claim 10 wherein said switch-operating device is anelectric heater and the switch operated thereby is a thermallyresponsive switch.